Optical communication networks often employ a technique, known as wavelength division multiplexing (WDM), to increase the amount of information that can be transmitted via the network. In WDM, multiple signals are transmitted through a communications path at distinct optical wavelengths. Each wavelength is intensity modulated by a different electrical signal and an optical multiplexer combines the modulated light beams into the communications path. At the receiver, an optical demultiplexer separates the different wavelengths and directs them to different detectors.
WDM systems are classified as either coarse or dense, depending on the number of channels and the channel spacing. Coarse WDM (CWDM) systems use a few channels with widely spaced wavelengths (20 nm or more). Dense WDM (DWDM) systems have a large number of channels (more than 8) with closely spaced wavelengths. Spacings of 0.2, 0.4 up to 1.6 nm are common. DWDM is used at higher levels in the communications hierarchy, but presents difficult design challenges.
An optical add-drop multiplexer (OADM) is an important routing device used in communication networks. In such a device, a demultiplexer sorts wavelength components, separates and drops one of the wavelength components, adds a new wavelength component, and combines all of the wavelength components back into a single beam using a multiplexer.